1. Field of the Invention
The present invention in one aspect relates to a camera shutter control. In another aspect the present invention relates to a camera having a single lens reflex view finder structure. More particularly, in this aspect the present invention concerns a camera having viewfinder optical means which is arranged to be retracted from an optical path at the time of picture taking and then returned to an original position in the optical path.
2. Description of the Related Art
Recently, there have been significant developments in connection with still video cameras which use an image pickup element such as a CCD to convert a light image into a corresponding electrical signal and to record the signal magnetically on a medium such as a video floppy disc.
In a still video camera, an automatic exposure (AE) control system of high precision is required in view of the narrowness of the dynamic range of the image pickup element. Thus, though it could be possible to control a stop value or a shutter speed based on an output of a photocell such as a silicon photocell (SPC) for measuring the object brightness, like in a film camera, it is difficult to construct an AE control system of high precision with that method due to the difference in the sensitivity and in the dynamic range between the image pickup element and the photocell.
In view of the above, there has been provided an AE control in which the image pickup element is first exposed to the image light for the purpose of determining the stop value or the shutter speed on the basis of the output of the photocell (a preparatory exposure). The stop value or the shutter speed is corrected based on the then obtained luminous output of the image pickup element to carry out the exposure of the pickup element for the image pickup (a true exposure). Thereafter the image pickup element is again exposed to the image light for the purpose of taking the picture.
FIG. 1 shows one example of such an AE control system. In the system shown in FIG. 1, a light measuring circuit 101 includes a photocell (SPC) and produces an output signal representing an object brightness. An information input circuit 102 produces information signals representing a set shutter speed (i.e. exposure time) or a set stop value (i.e. aperture size) and a full open lens F Number. In operation, circuit 103 is connected to receive the output signals from the circuits 101 and 102. A control circuit 104 is connected to receive the output signal from the operation circuit 103 and to control a diaphragm device 105 and a shutter device 106 based thereon. An image pickup circuit 107 includes an image pickup element (CCD) and produces an image signal and a luminous signal Y. A recording circuit 108 is connected to receive the output signal from the pickup circuit 107 and to record the signal on a medium such as a magnetic disc. The operation circuit 103 is connected to receive the luminous signal Y produced by the pickup circuit 107 and to connect the shutter speed and/or the stop value based thereon.
The operation circuit 103 determines an appropriate stop value or shutter speed on the basis of the output signals of the light measuring circuit 102 and of the information input circuit 102. Based on the output of the operation circuit 103, the control circuit 104 operates to stop down the diaphragm device 105 to the determined or set stop value and to open the shutter device 106 a certain period of time to expose the image pickup element (CCD) in the image pickup circuit 107 to the image light 109 (preparatory exposure). Thus, the pickup circuit 107 produces, together with the image signal, the luminous signal Y indicative of the object brightness. In response thereto, the operation circuit 103 corrects the set or determined shutter speed and/or the determined or set stop value. Then, the control circuit 104 operates the diaphragm device 105 and the shutter device 106 based on the corrected output of the operation circuit 103 to expose the pickup element (CCD) in the pickup circuit 107 under the most proper condition (true exposure). Thus, the most appropriate image signal is produced by the pickup circuit 107 and is recorded in the recording circuit 108.
In this system, the shutter device must be kept opened for a certain period of time for the preparatory exposure while it should be kept closed in an unused condition of the camera in order to protect the image pickup element from being damaged by the high intensity light etc.
However, if an electromagnetically driven focal plane type shutter device is used for the above mentioned shutter device, an electromagnet for driving a first one of shutter blades must be kept energized during the preparatory exposure to maintain the shutter device opened and this causes much consumption of electrical power.
Moreover, if a motion picture is to be taken with the above mentioned still video camera like a normal video camera, the shutter device should be kept opened for a whole period of the motion picture taking and this causes much more consumption of electrical power.
As is well known in the art, a single lens reflex type camera must have a mechanism for retracting a viewfinder mirror from an optical path at the time of picture taking and for thereafter returning the mirror to its original position in the optical path. Generally, such mechanism includes a spring as a driving source for quick movement of the mirror.
One well known film camera incorporates a manual or built in motor type automatic wind up mechanism; and the charging of the spring for mirror retraction is performed together with the film wind up by the wind up mechanism. Also, the releasing of a clutch for mirror return and the latching of the wind up mechanism are performed by the operation of a shutter.
However, in the case of a still video camera using a video floppy disc, the film wind up mechanism may be of no use and therefore it is not possible to charge the spring for mirror return by the film wind up mechanism.
Moreover, in the case of a still video camera, a very small and hence lightly operative shutter can be used because the picture size of the still video camera is about one fourth of that of a 35 mm film camera. Therefore, it may be very difficult to release the clutch for mirror return by the operation of such a small and lightly operative shutter.
Furthermore, it is not advisable to prepare an electro-magnet solely for the releasing of the clutch for mirror return.
Other than the above, it would be very convenient to take with the above mentioned still video camera, a motion picture like in a normal video camera, as well as a still picture. In this case, it would also be convenient to use an EVF (Electronic viewfinder) to monitor the image signal. Furthermore, it would be highly convenient if a VCR (Video Cassette Recorder) could be connected to the still video camera to record the picked up motion picture.